Kinetics and mechanism of hydroxyl radical reaction with methyl hydroperoxide (original) (raw)

1989, The Journal of Physical Chemistry

The reaction of hydroxyl radical with methyl hydroperoxide, CH300H, was investigated in the temperature range 203-423 K by pulsed photolytic generation of OH and detection by laser-induced fluorescence. The rate coefficient for the overall reaction, OH + C H 3 0 0 Hproducts ( k , ) was measured by using 180H and OD in place of OH. The rate coefficient for the CH3O2 production channel OH + CH3OOH -CH3O2 + H 2 0 (k,,) was obtained by using OH. The channel that yields CHzOOH, OH + C H 3 0 0 H -+ C H 2 0 0 H + H 2 0 (klb), is not observed when monitoring OH since C H 2 0 0 H rapidly falls apart to give back OH (and CH20) but is observed when studying the 180H or OD reaction with CH300H. By monitoring OH production in OD + C H 3 0 0 H reaction at 249 K, the two-channel mechanism was confirmed, and the values for k , and k , , were also determined. Both reaction 1 and channel la show negative activation energies, with k , = (2.93 & 0.30) X exp((220 & 21)/7') cm3 molecule-' s-I, where the indicated error is la, including estimated systematic errors and uA = AulnA. The rate coefficient for the reaction of OD with CH300D is at least a factor of 2 smaller than that for reaction la. The thermal decomposition lifetime for C H 2 0 0 H to give OH + CH20 is deduced to be shorter than 20 ws at 205 K. The mechanism of reaction 1 and the implications of our kinetic and mechanistic results to Earth's atmospheric chemistry are discussed. The measured value of kl and the branching ratio, kla/klb, at 298 K are compared with previous indirect measurements of Niki et al. [ J . Phys. Chem. 1983, 87, 21901. exp((l90 f 14)/7') cm3 molecule-' s-l (average of I80H and OD studies) and k,, = (1.78 & 0.25) X Phys. Lett. 1987, 139, 513. 7837 (9) Moortgat, G. K.; Burrows, J. P.; Schneider, W.; Tyndall, G. S.; Cox, R. A.